As the most popular method of noninvasively observing the interior of a human body and using the observation result in medical diagnosis, direct imaging of the transmittance distribution of X-rays transmitted through the human body is known. As such imaging method, a conventional method of imaging a fluorescence distribution due to X-rays that reach a phosphor using a silver halide film, a method of amplifying photoelectrons due to fluorescence using an image intensifier, and visualizing them using a TV camera, and a method of exciting latent image information formed by an X-ray intensity distribution on a photostimulable phosphor using a laser beam, and reading and visualizing the information are known. Furthermore, in recent years, a method of imaging the spatial distribution of free electrons generated in a heavy metal by fluorescence or X-ray radiation using a flat panel detector (FPD) which comprises a large-scale solid-state image sensing element that can cover the entire chest of a human body is put into practical applications due to the development of semiconductor technology.
The motions of a human body can be separated into parts such as a heart, gastro-intestinal, and the like that move depending on only the autonomic nerve, and parts such as respiration, four limbs, and the like that can also be moved intentionally. In conventional static image radiography, X-ray images are radiographed as static images since the part that can be moved intentionally can also be kept static intentionally. This is because a radiographic apparatus mainly uses a silver halide film or photostimulable phosphor. However, it is important to observe dynamic states of even such parts in the medical sense of the term. When such part is intentionally moved upon radiography without being kept static intentionally, since the rate of motion can be controlled, an exposure rate in tune with motion need not be set unlike that for the heart. For example, upon observing the respiratory behavior of a lung field, if a patient respires relatively slowly, the dynamic state can be satisfactorily observed even at an exposure rate of, e.g., about 3 frames per sec. However, it is difficult for a patient to intentionally move a given part too slowly, and respective parts have appropriate motion rates.
Conventionally, upon observing the dynamic states depending on the autonomic nerve, since such parts are limited ones, i.e., a heart, artery, gastro-intestinal, and the like, a required part can be radiographed using a TV camera system using an image intensifier. In this case, the exposure rate is important.
On the contrary, an imaging area is important in place of the exposure rate upon radiographing a part that can be moved intentionally, especially, upon radiographing the respiratory behavior of a lung field. For example, in order to observe a lung function from a dynamic state, various aspects from the motion of a diaphragm to that of veins inside the lung field must be observed. Therefore, the image intensifier with a limited imaging area cannot be used, and dynamic state observation must be done at a rate of about three frames per sec by exchanging large-sized films at high speed. However, in order to realize such radiography, a mechanism for exchanging large-sized films at high speed is required, and X-ray irradiation of a considerable dose is required to image films. For this reason, in order to obtain a plurality of images for dynamic state observation, huge dose and cost are required, and such method is far from practical use. Also, no method for optimally observing such plurality of images is available.
In recent years, along with the development of a large-sized X-ray FPD, the dynamic state of a human body over a relatively broad range (e.g., a chest) can be captured without any mechanical operations (e.g., those for exchanging films at high speed). However, even FPDs of practical use in terms of cost are those which have sensitivity equivalent to conventional films. Hence, the FPD does not have sensitivity as high as the TV system using the image intensifier, and requires a considerable dose to capture an image with sufficiently high image quality. For example, “J. H. Siewerdsen, L. E. Antonuk; DQE and System Optimization for Indirect-Detection Flat-Panel Imagers in Diagnostic Radiology; SPIE Vol. 3336”, a dose of X-rays that normally reach a sensor in chest static image radiography is 3 mR (milli-roentgens), and if a plurality of images for dynamic state observation are radiographed at this dose, a considerable dose is required.